FIG. 1 is a view showing the arrangement of the winding portion of a transformer. As shown in FIG. 1, an electric wire (winding) 3 is wound on a bobbin 1 made of an electric insulator. After that, the winding 3 is entirely or partly adhered to the bobbin 1 with an adhesive tape 2 so that the winding 3 may be reliably fixed to the bobbin 1. If the winding 3 has a sufficient number of turns, it can hold the bobbin 1; if the winding 3 has a small number of turns (e.g., one turn), it is difficult to fix the winding 3 to the bobbin 1. For this reason, it is very important to adhere the winding 3 to the bobbin 1 with the adhesive tape 2 or the like.
To adhere the winding 3 with the adhesive tape 2, the bobbin 1 must have a wide winding space. Consequently, a bobbin 1 having a large winding space is necessary, sometimes leading to an increase in the size of the transformer. Also, the adhering process using the adhesive tape 2 increases the manufacturing cost of the transformer.
The terminals of the bobbin 1 to which two terminal end portions 5 of the winding 3 are to be connected are generally arranged in the vicinity of the bottom surface of the bobbin 1 which forms a hollow prism or cylinder. Hence, after winding, extracting portions 4 of the winding 3 must be pulled out in directions largely different from the winding direction. Accordingly, for example, when a plurality of coils are to be formed on the bobbin 1, a plurality of extracting portions 4 occupy the winding space of the bobbin 1. Formation of the extracting portions 4 obviously complicates the winding process of the winding 3.
To decrease the resistance of the coil, sometimes a plurality of coils are formed in one layer on the bobbin 1 and are connected parallel to each other. If, however, the winding 3 is far from the terminals to which the two terminal end portions 5 are to be connected, its extracting portions 4 become long. Particularly, when the winding 3 has a small number of turns (e.g., one turn), the proportion of the extracting portions 4 in the entire length of the winding 3 becomes large. If the plurality of coils are formed in one layer, the differences in electric wire length among the coils become conspicuous. Therefore, even when the coils are connected parallel to each other, the total resistance of the coils does not decrease so much for the number of parallel coils. Also, due to the differences in resistance, the current values among the coils differ.